In a cellular system, handover control is carried out to appropriately switch cells (base stations) to which a user is connected along with movement of the user. In the handover control, propagation conditions in peripheral cells are measured by a mobile station, and the handover is carried out in accordance with the measurement result, in order to realize the handover to an appropriate adjacent cell.
In this case, the peripheral cells and the cell to which the mobile station is currently connected may operate on different frequencies, or on plural frequencies. When the handover is to be carried out to a cell using different frequencies, the mobile station needs to measure the propagation conditions in different frequencies in the peripheral cells and the current cell.
In addition, a handover to a peripheral cell that employs a different radio transmission method (different system) may be advantageous in terms of a traffic amount and propagation conditions. In this situation, the mobile station needs to measure different frequencies and systems during communications.
It should be noted here that a mobile station that has only one receiver cannot concurrently measure plural frequencies and systems.
This is because an REF (Radio Frequency) circuit of the receiver cannot be concurrently attuned to plural frequency carriers or systems. For plural frequencies or systems to be concurrently measured, the mobile station has to include plural receivers (RF circuits), which increases the size, energy consumption, and price of the mobile station.
Therefore, the mobile stations currently widely used have only one receiver. When such mobile stations measure the different frequencies and the different systems, the currently-conducted communications are switched to Discontinuous Reception (DRX), and a gap period caused during the DRX is used for the measurement. In this case, unless the base station recognizes the gap period of the DRX, the base station may transmit data during the gap time when the mobile station is carrying out the measurement. Such transmission of the data may not only lead to a waste of precious radio resources but also cause adverse effects such as increased interference power in other communications and lengthened delay. In order to avoid such transmissions, the base station has to recognize the DRX status of the mobile station.
In a radio communications system such as HSDPA, the mobile station frequently notifies the base station of radio channel conditions (referred to as Channel Quality Indicator (CQI)) in order to carry out link adaptation that tracks fast fading, for example, Transmission Power Control and Adaptive Modulation and Coding (AMC). For example, the mobile station measures the chip energy to total received power (Ec/Io) of a common pilot channel transmitted by the base station, and sends a value obtained by quantizing the Ec/Io into 32 levels as the CQI in a time cycle of 2 ms (or 2× integer ms), in HSDPA.